This invention relates to an improvement of a display device utilizing electrochromic phenomenon.
Certain substances are colored if electric current flows therethrough and the coloring of this fashion is called the "electrochromic phenomenon." On the other hand, the substance exhibiting the electrochromic phenomenon is called the "electrochromic material." This coloring is bleached by the flow of current in the reverse direction. In general, the electrochromic phenomenon is thought to accompany the formation of color centers or the oxidation-reduction reaction.
British Pat. No. 1,186,541 and U.S. Pat. No. 3,807,832 disclose many electrochromic materials. Tungsten oxide and molybdenum oxide are typical examples of inorganic electrochromic materials. Organic electrochromic materials such as viologen bromide are also under development.
Described in the following is an example of the conventional electrochromic display device, with reference to the appended FIG. 1. In the drawing, reference numeral 1 denotes a tungsten oxide film providing an electrochromic material. It is seen that the electrochromic material film 1 is formed on a transparent or translucent conductive film 2 acting as a conductive electrode. Further, the conductive film 2 is formed on a transparent substrate 3. Reference numeral 4 represents a counter electrode formed about 1 mm apart from the substrate 3. As shown in the drawing, an electrolyte 5 is sealed by a sealing material 6 between the film 1 and the counter electrode 4. The electrochromic material film 1 is colored blue when voltage is applied across the conductive film 2 and the counter electrode 4 such that the former acts as the negative electrode and the latter as the positive electrode. The coloring is observed, for example, in the direction shown by an arrow 7. As described previously, the coloring is bleached if the reverse voltage is applied across the film 2 and the counter electrode 4.
Capability of a rapid response is one of the properties required for the electrochromic display device. Namely, the device should be capable of a rapid coloration and a rapid bleach. The idea of enlarging the surface area of the electrochromic material film has already been proposed as a means for achieving a rapid response. For example. Japanese patent disclosure (Kokai) No. 51-23100 (disclosed on Feb. 24, 1976) discloses an electrochromic display device having a tungsten oxide layer with many holes and capable of a rapid coloration and rapid bleach. In this case the electrochromic material layer is formed by means of vapor deposition on a conductive film with a mesh interposed therebetween, thereby obtaining the electrochromic material layer with many holes.
FIG. 2 shows a modification of FIG. 1. Namely, a electrochromic material layer 21 of tungsten oxide with many holes is substituted in FIG. 2 for the tungsten oxide film 1 shown in FIG. 1. Incidentally, the reference numerals of FIG. 2, which are the same as those of FIG. 1, denote the same members. FIG. 2 clearly shows that the electrolyte 5 is in contact with the tungsten oxide layer 21 and also contacts with the conductive film 2 through the holes of the electrochromic material layer 21. It is important to note that tin oxide, which is readily corroded by the electrolyte such as sulfuric acid, is used as a typical material of the conductive film 2. To be more specific, tin oxide is gradually reduced into tin and water by the hydrogen ion attracted to the conductive film during the voltage application. It follows that the conductivity of the conductive film is lowered, leading to the impaired performance of the display device.